1. Field of the Disclosure
The present disclosure relates to a mixed dispersant, and a paste composition and a dispersion method using the same, and more particularly, to a mixed dispersant which improves the efficiency of dispersing a metal powder by effectively adsorbing on the surface of the metal powder and preventing aggregation of the metal powder, and a paste composition and a dispersion method using the same. The present invention also relates to a multilayer ceramic capacitor (MLCC).
2. Description of the Related Art
A multilayer ceramic condenser (hereinafter, referred to as MLCC) is produced by laminating a plurality of dielectric thin layers and a plurality of internal electrodes. An MLCC with this structure has a large capacitance relative to its small volume, and thus, is widely used in a variety of electronic appliances such as, for example, personal computers and mobile telecommunication devices.
A silver-palladium (Ag—Pd) alloy has been used as the material for the internal electrode, which constitutes the MLCC. The silver-palladium alloy can be easily applied to the production of MLCCs since the alloy can be sintered even in the atmospheric air; however, the alloy is expensive and the economics are not favorable. In order to lower the cost of the MLCCs, there was an attempt in the latter half of 1990's to replace the silver-palladium alloy with inexpensive nickel for the material of the internal electrode. Accordingly, nickel electrodes are increasingly used as the internal electrodes of the MLCCs, and in this instance, the nickel internal electrodes are formed from a conductive paste containing nickel metal powder.
The nickel metal powder can be prepared by various preparative processes, and representative processes include a gas phase process and a liquid phase process. The gas phase process is widely used since it is relatively easy to control the morphology of the nickel metal powder and the presence of impurities, yet the process is disadvantageous in the aspects of particle size reduction and mass production. On the other hand, the liquid phase process is advantageous in that mass production is possible and the costs for facilities installation and operation maintenance are low, thus the process being used with favor. The liquid phase process is described in, for example, U.S. Pat. Nos. 4,539,041 and 6,120,576.
However, even when the nickel metal powder is produced by the liquid phase process or the gas phase process, upon the use of the nickel metal powder for the preparation of a conductive paste composition, a large quantity of the nickel metal powder cannot be used because the viscosity of the paste composition may be excessively high. Therefore, a method is known in which the nickel metal powder is dispersed in the paste composition by means of various kinds of dispersants. A dispersant in general exhibits its dispersing ability by adsorbing on the surface of metal powder and suppressing aggregation of the powder. Thus, in order to facilitate adsorption of the dispersant, a dispersant having a functional group which is effective for adsorption, that is, an acidic dispersant, has been used for the nickel metal powder that is basic in nature, so as to disperse the nickel metal powder in the paste. However, there remains a demand for a dispersant having an improved dispersing ability to achieve a satisfactory efficiency of dispersion and to increase the amount of the nickel metal powder contained in the paste composition.